The present invention relates to a method of plugging a rotating welding tool into a weld zone of workpieces to perform friction stir welding, and a friction stir welding apparatus.
Methods of solid-state welding two workpieces without melting include a welding method making use of a friction stir phenomenon caused by rotation of a welding tool, formed from a harder material than the workpieces, by rotating and plugging the welding tool into a weld zone of the workpieces and moving the welding tool along a welding direction, such method being called friction stir welding.
Included among friction stir welding is one, in which an external heat source such as laser or the like preheats a weld zone and its neighborhood (for example, see Patent Document 1).    Patent Document 1: JP-A-10-225781 (Abstract, claim 1, FIG. 1)
In the case where temperature of a weld zone is raised only by friction heat generated between a welding tool and workpieces in friction stir welding, a very large friction force is generated in friction stir welding of thick plates or the like. When a friction force is large, workpieces become liable to be deformed, and a welding tool is shortened in life. Even when preheating is effected from outside by laser or the like, a friction force almost accounts for raising temperature of a weld zone. Therefore, a large friction force acts on a welding tool or workpieces as reaction thereof.
The invention has its object to provide a method capable of decreasing generation of a friction force in friction stir welding, and an apparatus therefor.